This invention relates to a mobile radio telephone system having a plurality of base stations.
The purpose of a mobile radio telephone system is to enable a network such as a public switched telephone network to be accessed from a mobile station such as an automobile. Such a system comprises a plurality of zones each having a base station for communication with mobile stations in that zone. It is desirable for the system to have a hand-off function for switching communication from one base station to another as a mobile station moves from one zone to another.
FIG. 1 shows the configuration of a prior-art mobile radio telephone system having a hand-off function, in which the reference numeral 1 denotes a line control unit, 2 is a line of, for example, a public switched telephone network, A, B, and C are cellular zones with base stations 5A, 5B, and 5C, and 6 is a mobile station, and 7 is its mobile antenna. The base stations 5A, 5B, and 5C comprise base station control units 11a to 11c, control channel transceivers 12a to 12c, voice channel transceivers 13a to 13c, locator receivers 14a to 14c, and base station antennas 15a to 15c.
The call hand-off function operates as follows. Separate control channels are allocated to the control channel transceivers 12a to 12c in each cellular zone A, B, and C. Normally system numbers and idle channel signals are transmitted on the control channels in each zone A, B, and C. A mobile station 6 scans all the control channels. When it is in cellular zone B, for example, it receives the control channel signal broadcast from the base station 5B, locks onto this channel, and waits in a standby mode. When the mobile station 6 originates a call, it sends a call request signal on the control channel to which it is currently locked. The call request signal is received by the antenna 15b and the control channel transceiver 12b and transmitted through the base station control unit 11b to the line control unit 1.
The base station control unit 11b allocates an idle voice channel of the voice channel transceiver 13b to the mobile station 6, then sends the mobile station 6 a channel switching signal via the control channel transceiver 12b, instructing the mobile station 6 to switch to the allocated voice channel.
The line control unit 1 decodes the call request signal from the mobile station 6, sends a dialing signal to the telephone line 2, and connects the voice channel of the base station 5B to the telephone line 2. When the answering party on the telephone line 2 goes off-hook, communication is established between the mobile station 6 and the answering party.
If the mobile station 6 moves out of the cellular zone B during the call, the signal quality on the voice channel is degraded. The degradation is detected by the voice channel transceiver 13b, which sends a hand-off request to the line control unit 1 via the base station control unit 11b. The line control unit 1 sends a field measurement request to the base stations, such as the base stations 5A and 5C, adjacent to the base station 5B. Using their locator receivers 14a and 14c, these base stations measure and report the received field intensity on the voice channel allocated to the base station 5B. The line control unit 1 switches the voice channel of the base station reporting the highest measured field intensity, the base station 5A for example, to the channel of the mobile station 6, and switches the telephone line 2 from the base station 5B to the base station 5A, thus handing off the call from the base station 5B to the base station 5A. When the telephone line 2 or the mobile station 6 terminates the call by going on-hook, the base station 5A or the mobile station 6 sends a call end signal and the mobile station 6 returns to the standby mode on the control channel.
A drawback of the system illustrated in FIG. 1 is that the call hand-off procedure is overly complex, and requires each base station to have a locator receiver.
FIG. 2 shows another example of the prior art, being a block diagram of the base station apparatus described in an article, New Cordless Telephone System in Equipment on Telecommunication, a periodical publication of the Telecommunication Association, Vol. 36, No. 3, pp. 134-141, 1979. The apparatus shown in FIG. 2 comprises a transmitter 21, a receiver 22, a synthesizer oscillator 23, a transmitting antenna 24 for the transmitter 21, a receiving antenna 25 for the receiver 22, an idle channel and interference detector 26, a control section 27, a hybrid transformer 28, a telephone line 29, and a mobile station 30. The control section 27 combines the functions of the line control unit and base station control unit in the preceding example.
The apparatus in FIG. 2 operates as follows. The transmitter 21 and the receiver 22 are driven by the same synthesizer oscillator 23. Normally the control section 27 keeps the transmitter 21 and the receiver 22 connected to the control channel, but during a call these are switched to the voice channel. The receiver 22 is connected to the idle channel and interference detector 26, the detection results of which are also supplied to the control section 27.
When the mobile station 30 originates a call on the control channel, the receiving antenna 25 and the receiver 22 receive the calling signal and pass it to the control section 27, which decodes it. The control section 27 enables the idle channel and interference detector 26, switches the transmitter 21 and the receiver 22 to a voice channel, and performs an internal check to ascertain whether another mobile station is using the voice channel. If an idle voice channel can be found by the idle channel detector, the mobile station 30 sends an address signal that is received by the control section 27, and then transmitted by the line control unit in the control section 27 to the telephone line 29. This system is simple because each base station is independent and no separate line control unit is required, but this system has no call hand-off function and makes no provision for the case in which a call from a mobile station 30 is received by two or more base stations.
In the base station apparatus of the cordless telephone system in FIG. 2, the receiver 22 is normally in a standby mode waiting for a call from a mobile station 30 on the control channel, and only switches over to the voice channel when a call is set up. A problem is that if two or more base stations are located proximately, a calling signal intended for only one of the base stations may be received by two or more base stations that are standing by on the control channel. To ensure that the calling signal is received by a particular base station, the control section 27 must have an ID (Identification Data) which is assigned individually to each base station, in which case each base station operates as a distinct system, or the mobile station 30 must register its own ID with a particular base station in advance and specify that base station when originating a call. Such requirements, however, detract from the freedom of use of the system.